Method for performing device-to-device direct communication in wireless communication system and device therefor

ABSTRACT

Disclosed are a direct communication method and ProSe-enabled UE, the direct communication method receiving an identifier of MME supporting relay UE from the relay UE in which a direct connection to remote UE is established, and transmitting, to a base station supporting the relay UE, a TAU request message for requesting the MME supporting the relay UE to manage context information of the remote UE together, and the identifier of MME.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for efficiently managing acontext through device-to-device (D2D) direct communication in a D2Ddirect communication (e.g., ProSe communication) environment.

BACKGROUND ART

Wireless access systems have been widely deployed to provide varioustypes of communication services such as voice or data. In general, awireless access system is a multiple access system that may supportcommunication of multiple users by sharing available system resources(e.g., a bandwidth, transmission power, etc.). For example, multipleaccess systems include a Code Division Multiple Access (CDMA) system, aFrequency Division Multiple Access (FDMA) system, a Time DivisionMultiple Access (TDMA) system, an Orthogonal Frequency Division MultipleAccess (OFDMA) system, a Single Carrier Frequency Division MultipleAccess (SC-FDMA) system, and a multi carrier frequency division multipleaccess (MC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Task

An object of the present invention is to improve a method performed by anetwork entity for supporting D2D direct communication in a ProSecommunication process.

Another object of the present invention is to reduce power consumptionby establishing a relationship among a plurality of coexisting userequipments.

A further object of the present invention is to achieve efficientmobility management by controlling contexts of user equipments.

The technical problems solved by the present invention are not limitedto the above technical problems and other technical problems which arenot described herein will become apparent to those skilled in the artfrom the following description.

Technical Solutions

To achieve these objects and other advantages, in an aspect of thepresent invention, provided herein is a method for performing directcommunication, including: receiving an identifier of a first mobilitymanagement entity (MME), which is an MME supporting a relay userequipment (UE), from the relay UE having a direct connection with aremote UE; and transmitting a tracking area update (TAU) request messagefor requesting the first MME supporting the relay UE to manage contextinformation of the remote UE together and the identifier of the firstMME to an evolved node B (eNB) supporting the relay UE.

The identifier of the first MME may be received through a PC5 messagefrom the relay UE, and a non-access stratum (NAS) layer of the remote UEmay forward the identifier of the first MME to an access stratum (AS)layer. The TAU request message and the identifier of the first MME maybe transmitted to the eNB through a radio resource control (RRC)message, and the eNB may identify the first MME to which the TAU requestmessage will be forwarded using the identifier of the first MME.

Receiving may include receiving, from the relay UE, a systemarchitecture evolution (SAE) temporary mobile subscriber identity(S-TMSI) of the relay UE instead of the identifier of the first MME. Anon-access stratum (NAS) layer of the remote UE forwards the identifierof the first MME or the S-TMSI of the relay UE to an access stratum (AS)layer, and the S-TMSI of the relay UE may be forwarded to the eNBthrough a radio resource control (RRC) message.

Receiving may include receiving an identifier (ID) of a public landmobile network in which the relay UE is currently registered and an IDof a serving cell of the relay UE together with the identifier of thefirst MME, and the remote UE may perform a PLMN alignment process and acell alignment process using the PLMN ID and the serving cell ID beforetransmitting the TAU request message

The relay UE and the remote UE may form a prescribed UE group wheremobility is managed together. In this case, the relay UE may be arepresentative UE of the UE group, and the remote UE may be asubordinary UE of the UE group.

If an MME is changed while a TAU procedure for the relay UE isperformed, context information of the relay UE and the contextinformation of the remote UE may be transmitted together from a previousMME to a new MME.

When paging of the remote UE is detected, the relay UE may forward apaging message to the remote UE through the direct connection with theremote UE.

When the TAU request message is received, the first MME may obtain thecontext information of the remote UE from a second MME, which is an MMEsupporting the remote UE.

In another aspect of the present invention, provided herein is a relayuser equipment (UE), including: a transmitter; a receiver; and aprocessor connected to the transmitter and the receiver. In this case,the processor may be configured to receive an identifier of a firstmobility management entity (MME), which is an MME supporting the relayUE, from the relay UE having a direct connection with the remote UE andtransmit a tracking area update (TAU) request message for requesting thefirst MME supporting the relay UE to manage context information of theremote UE together and the identifier of the first MME to an evolvednode B (eNB) supporting the relay UE.

Advantageous Effects

As is apparent from the above description, the embodiments of thepresent invention have the following effects.

First, a method for supporting D2D direct communication can be improved,thereby preventing network entities from wasting radio resources.

Second, when a user uses a plurality of UEs, power consumption of theUEs can be efficiently improved.

Third, it is possible to reduce signaling overhead required for UEmobility management by controlling contexts between network entities.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinmay be derived by those skilled in the art from the followingdescription of the embodiments of the present invention. That is,effects which are not intended by the present invention may be derivedby those skilled in the art from the embodiments of the presentinvention.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention. The technical features of the present invention are notlimited to specific drawings and the features shown in the drawings arecombined to construct a new embodiment. Reference numerals of thedrawings mean structural elements.

FIG. 1 is a diagram illustrating a brief structure of an EPS (evolvedpacket system) that includes an EPC (evolved packet core);

FIG. 2 is an exemplary diagram illustrating an architecture of a generalE-UTRAN and a general EPC;

FIG. 3 is an exemplary diagram illustrating a structure of a radiointerface protocol on a control plane;

FIG. 4 is an exemplary diagram illustrating a structure of a radiointerface protocol on a user plane;

FIG. 5 is a flow chart illustrating a random access procedure;

FIG. 6 is a diagram illustrating a connection procedure in a radioresource control (RRC) layer;

FIG. 7 illustrates a basic path for communication between two UEs in theEPS;

FIG. 8 illustrates a ProSe-based direct-mode communication path betweentwo UEs;

FIG. 9 illustrates a ProSe-based communication path between two UEsthrough an eNodeB;

FIG. 10 illustrates a non-roaming reference architecture;

FIG. 11 is a diagram illustrating communication through a ProseUE-to-Network Relay;

FIG. 12 is a diagram illustrating media traffic of group communication;

FIG. 13 is a diagram illustrating a procedure in which a remote UEperforms direct communication through a UE-to-network relay;

FIG. 14 illustrates a case in which an SGW is changed together with atracking area update (TAU) procedure;

FIGS. 15 to 17 illustrate direct communication methods according toproposed embodiments; and

FIG. 18 is a diagram illustrating the configuration of a node deviceaccording to a proposed embodiment.

BEST MODE FOR INVENTION

Although the terms used in the present invention are selected fromgenerally known and used terms, terms used herein may be varieddepending on operator's intention or customs in the art, appearance ofnew technology, or the like. In addition, some of the terms mentioned inthe description of the present invention have been selected by theapplicant at his or her discretion, the detailed meanings of which aredescribed in relevant parts of the description herein. Furthermore, itis required that the present invention is understood, not simply by theactual terms used but by the meanings of each term lying within.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments or may be replaced with those of the other embodiments asnecessary.

In describing the present invention, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present invention unnecessarily ambiguous, the detaileddescription thereof will be omitted.

In the entire specification, when a certain portion “comprises orincludes” a certain component, this indicates that the other componentsare not excluded and may be further included unless specially describedotherwise. The terms “unit”, “-or/er” and “module” described in thespecification indicate a unit for processing at least one function oroperation, which may be implemented by hardware, software or acombination thereof. The words “a or an”, “one”, “the” and words relatedthereto may be used to include both a singular expression and a pluralexpression unless the context describing the present invention(particularly, the context of the following claims) clearly indicatesotherwise.

The embodiments of the present invention can be supported by thestandard documents disclosed in any one of wireless access systems, suchas an IEEE 802.xx system, a 3rd Generation Partnership Project (3GPP)system, a 3GPP Long Term Evolution (LTE)/LTE-Advanced (LTE-A) system,and a 3GPP2 system. That is, the steps or portions, which are notdescribed in order to make the technical spirit of the present inventionclear, may be supported by the above documents.

In addition, all the terms disclosed in the present document may bedescribed by the above standard documents. In particular, theembodiments of the present invention may be supported by at least one ofP802.16e-2004, P802.16e-2005, P802.16.1, P802.16p and P802.16.1bdocuments, which are the standard documents of the IEEE 802.16 system.

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description which will be disclosed alongwith the accompanying drawings is intended to describe the exemplaryembodiments of the present invention and is not intended to describe aunique embodiment which the present invention can be carried out.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

Terms used in the specification are defined as follows.

-   -   UMTS (Universal Mobile Telecommunications System): a GSM (Global        System for Mobile Communication) based third generation mobile        communication technology developed by the 3GPP.    -   EPS (Evolved Packet System): a network system that includes an        EPC (Evolved Packet Core) which is an IP (Internet Protocol)        based packet switched core network and an access network such as        LTE and UTRAN. This system is the network of an evolved version        of the UMTS.    -   NodeB: a base station of GERAN/UTRAN. This base station is        installed outdoor and its coverage has a scale of a macro cell.    -   eNodeB: a base station of LTE. This base station is installed        outdoor and its coverage has a scale of a macro cell.    -   UE (User Equipment): the UE may be referred to as terminal, ME        (Mobile Equipment), MS (Mobile Station), etc. Also, the UE may        be a portable device such as a notebook computer, a cellular        phone, a PDA (Personal Digital Assistant), a smart phone, and a        multimedia device. Alternatively, the UE may be a non-portable        device such as a PC (Personal Computer) and a vehicle mounted        device. The term “UE”, as used in relation to MTC, can refer to        an MTC device.    -   HNB (Home NodeB): a base station of UMTS network. This base        station is installed indoor and its coverage has a scale of a        micro cell.    -   HeNB (Home eNodeB): a base station of an EPS network. This base        station is installed indoor and its coverage has a scale of a        micro cell.    -   MME (Mobility Management Entity): a network node of an EPS        network, which performs mobility management (MM) and session        management (SM).    -   PDN-GW (Packet Data Network-Gateway)/PGW/P-GW: a network node of        an EPS network, which performs UE IP address allocation, packet        screening and filtering, charging data collection, etc.    -   SGW (Serving Gateway/S-GW: a network node of an EPS network,        which performs mobility anchor, packet routing, idle-mode packet        buffering, and triggering of an MME's UE paging.    -   PCRF (Policy and Charging Rule Function): a network node of an        EPS network, which performs a policy decision to dynamically        apply different QoS and charging policies for each service flow.    -   OMA DM (Open Mobile Alliance Device Management): a protocol        designed to manage mobile devices such as a cell phone, a PDA,        and a laptop computer, which performs functions such as device        configuration, firmware upgrade, error report, and the like.    -   OAM (Operation Administration and Maintenance): a set of network        management functions, which provides network error display,        performance information, data, and management functions.    -   NAS (Non-Access Stratum): a higher stratum of a control plane        between a UE and MME. As a functional layer for exchanging        signaling and traffic messages between a UE and core network in        LTE/UMTS protocol stack, the NAS supports UE mobility, a session        management procedure for establishing and maintaining an IP        connection between a UE and PDN GW, and IP address management.    -   AS (Access-Stratum): the AS includes a protocol stack between a        UE and a radio (or access) network, which manages transmission        of data and network control signals.    -   NAS configuration MO (Management Object): the NAS configuration        MO is a management object (MO) used to configure parameters        related to NAS functionality for a UE.    -   PDN (Packet Data Network): a network in which a server        supporting a specific service (e.g., a Multimedia Messaging        Service (MMS) server, a Wireless Application Protocol (WAP)        server, etc.) is located.    -   PDN connection: a logical connection between a UE and a PDN,        represented as one IP address (one IPv4 address and/or one IPv6        prefix).    -   APN (Access Point Name): a character string for indicating or        identifying PDN. To access a requested service or network, a        connection to a specific P-GW is required. The APN means a name        (character string) predefined in a network to search for the        corresponding P-GW (for example, it may be defined as        internet.mnc012.mcc345.gprs).    -   RAN (Radio Access Network): a unit including a Node B, an eNode        B, and a Radio Network Controller (RNC) for controlling the Node        B and the eNode B in a 3GPP network, which is present between        UEs and provides a connection to a core network.    -   HLR (Home Location Register)/HSS (Home Subscriber Server): a        database having subscriber information in a 3GPP network. The        HSS can perform functions such as configuration storage,        identity management, and user state storage.    -   PLMN (Public Land Mobile Network): a network configured for the        purpose of providing mobile communication services to        individuals. This network can be configured per operator.    -   ANDSF (Access Network Discovery and Selection Function): This is        one of network entities for providing a policy for discovering        and selecting an access that can be used by a UE on an operator        basis.    -   Proximity Services (or ProSe Service or Proximity-based        Service): a service that enables discovery between physically        proximate devices, and mutual direct communication/communication        through a base station/communication through the third party. At        this time, user plane data are exchanged through a direct data        path without through a 3GPP core network (for example, EPC).    -   ProSe Communication: communication between two or more        ProSe-enabled UEs in proximity by means of a ProSe Communication        path. Unless explicitly stated otherwise, the term “ProSe        Communication” refers to any/all of the following: ProSe E-UTRA        Communication, ProSe-assisted WLAN direct communication between        two UEs, ProSe Group Communication and ProSe Broadcast        Communication.    -   ProSe E-UTRA Communication: ProSe Communication using a ProSe        E-UTRA Communication path.    -   ProSe-assisted WLAN direct communication: ProSe Communication        using a ProSe-assisted WLAN direct communication path.    -   ProSe Communication path: communication path supporting ProSe        Communication. The ProSe E-UTRA Communication path could be        established between the ProSe-enabled UEs using E-UTRA, or        routed via local eNB(s). The ProSe-assisted WLAN direct        communication path may be established directly between the        ProSe-enabled UEs using WLAN.    -   EPC Path (or infrastructure data path): the user plane        communication path through EPC.    -   ProSe Discovery: a process that identifies that a UE that is        ProSe-enabled is in proximity of another, using E-UTRA.    -   ProSe Group Communication: one-to-many ProSe Communication,        between more than two ProSe-enabled UEs in proximity, by means        of a common communication path established between the        ProSe-enabled UEs.    -   ProSe UE-to-Network Relay: is a form of relay in which a        ProSe-enabled Public Safety UE acts as a communication relay        between a ProSe-enabled Public Safety UE and the ProSe-enabled        network using E-UTRA.    -   Remote UE: This is a ProSe-enabled Public Safety UE that is        connected to an EPC network through a ProSe UE-to-network relay        instead of being served by an E-UTRAN in UE-to-Network Relay        operation. That is, a PDN connection is provided to the remote        UE.    -   ProSe-enabled Network: a network that supports ProSe Discovery,        ProSe Communication and/or ProSe-assisted WLAN direct        communication. Hereinafter, the ProSe-enabled Network may simply        be referred to as a network.    -   ProSe-enabled UE: a UE that supports ProSe Discovery, ProSe        Communication and/or ProSe-assisted WLAN direct communication.        Hereinafter, the ProSe-enabled UE and the ProSe-enabled Public        Safety UE may be referred to as UE.    -   Proximity: proximity is determined (“a UE is in proximity of        another UE”) when given proximity criteria are fulfilled.        Proximity criteria can be different for discovery and        communication.

1. Evolved Packet Core (EPC)

FIG. 1 is a schematic diagram showing the structure of an evolved packetsystem (EPS) including an evolved packet core (EPC).

The EPC is a core element of system architecture evolution (SAE) forimproving performance of 3GPP technology. SAE corresponds to a researchproject for determining a network structure supporting mobility betweenvarious types of networks. For example, SAE aims to provide an optimizedpacket-based system for supporting various radio access technologies andproviding an enhanced data transmission capability.

Specifically, the EPC is a core network of an IP mobile communicationsystem for 3GPP LTE and can support real-time and non-real-timepacket-based services. In conventional mobile communication systems(i.e. second-generation or third-generation mobile communicationsystems), functions of a core network are implemented through acircuit-switched (CS) sub-domain for voice and a packet-switched (PS)sub-domain for data. However, in a 3GPP LTE system which is evolved fromthe third generation communication system, CS and PS sub-domains areunified into one IP domain. That is, in 3GPP LTE, connection ofterminals having IP capability can be established through an IP-basedbusiness station (e.g., an eNodeB (evolved Node B)), EPC, and anapplication domain (e.g., IMS). That is, the EPC is an essentialstructure for end-to-end IP services.

The EPC may include various components. FIG. 1 shows some of thecomponents, namely, a serving gateway (SGW), a packet data networkgateway (PDN GW), a mobility management entity (MME), a serving GPRS(general packet radio service) supporting node (SGSN) and an enhancedpacket data gateway (ePDG).

The SGW operates as a boundary point between a radio access network(RAN) and a core network and maintains a data path between an eNodeB andthe PDN GW. When. When a terminal moves over an area served by aneNodeB, the SGW functions as a local mobility anchor point. That is,packets. That is, packets may be routed through the SGW for mobility inan evolved UMTS terrestrial radio access network (E-UTRAN) defined after3GPP release-8. In addition, the SGW may serve as an anchor point formobility of another 3GPP network (a RAN defined before 3GPP release-8,e.g., UTRAN or GERAN (global system for mobile communication(GSM)/enhanced data rates for global evolution (EDGE) radio accessnetwork).

The PDN GW corresponds to a termination point of a data interface for apacket data network. The PDN GW may support policy enforcement features,packet filtering and charging support. In addition, the PDN GW may serveas an anchor point for mobility management with a 3GPP network and anon-3GPP network (e.g., an unreliable network such as an interworkingwireless local area network (I-WLAN) and a reliable network such as acode division multiple access (CDMA) or WiMax network).

Although the SGW and the PDN GW are configured as separate gateways inthe example of the network structure of FIG. 1, the two gateways may beimplemented according to a single gateway configuration option.

The MME performs signaling and control functions for supporting accessof a UE for network connection, network resource allocation, tracking,paging, roaming and handover. The MME controls control plane functionsassociated with subscriber and session management. The MME managesnumerous eNodeBs and signaling for selection of a conventional gatewayfor handover to other 2G/3G networks. In addition, the MME performssecurity procedures, terminal-to-network session handling, idle terminallocation management, etc.

The SGSN handles all packet data such as mobility management andauthentication of a user for other 3GPP networks (e.g., a GPRS network).

The ePDG serves as a security node for a non-3GPP network (e.g., anI-WLAN, a Wi-Fi hotspot, etc.).

As described above with reference to FIG. 1, a terminal having IPcapabilities may access an IP service network (e.g., an IMS) provided byan operator via various elements in the EPC not only based on 3GPPaccess but also on non-3GPP access.

Additionally, FIG. 1 shows various reference points (e.g. S1-U, S1-MME,etc.). In 3GPP, a conceptual link connecting two functions of differentfunctional entities of an E-UTRAN and an EPC is defined as a referencepoint. Table 1 is a list of the reference points shown in FIG. 1.Various reference points may be present in addition to the referencepoints in Table 1 according to network structures.

TABLE 1 Reference point Description S1-MME Reference point for thecontrol plane protocol between E-UTRAN and MME S1-U Reference pointbetween E-UTRAN and Serving GW for the per bearer user plane tunnelingand inter eNodeB path switching during handover S3 It enables user andbearer information exchange for inter 3GPP access network mobility inidle and/or active state. This reference point can be used intra-PLMN orinter-PLMN (e.g. in the case of Inter-PLMN HO). S4 It provides relatedcontrol and mobility support between GPRS Core and the 3GPP Anchorfunction of Serving GW. In addition, if Direct Tunnel is notestablished, it provides the user plane tunneling. S5 It provides userplane tunneling and tunnel management between Serving GW and PDN GW. Itis used for Serving GW relocation due to UE mobility and if the ServingGW needs to connect to a non-collocated PDN GW for the required PDNconnectivity. S11 Reference point between an MME and an SGW SGi It isthe reference point between the PDN GW and the packet data network.Packet data network may be an operator external public or private packetdata network or an intra operator packet data network, e.g. forprovision of IMS services. This reference point corresponds to Gi for3GPP accesses.

Among the reference points shown in FIG. 1, S2 a and S2 b correspond tonon-3GPP interfaces. S2 a is a reference point which provides reliablenon-3GPP access and related control and mobility support between PDN GWsto a user plane. S2 b is a reference point which provides relatedcontrol and mobility support between the ePDG and the PDN GW to the userplane.

FIG. 2 is a diagram exemplarily illustrating architectures of a typicalE-UTRAN and EPC.

As shown in the figure, while radio resource control (RRC) connection isactivated, an eNodeB may perform routing to a gateway, schedulingtransmission of a paging message, scheduling and transmission of abroadcast channel (BCH), dynamic allocation of resources to a UE onuplink and downlink, configuration and provision of eNodeB measurement,radio bearer control, radio admission control, and connection mobilitycontrol. In the EPC, paging generation, LTE_IDLE state management,ciphering of the user plane, SAE bearer control, and ciphering andintegrity protection of NAS signaling.

FIG. 3 is a diagram exemplarily illustrating the structure of a radiointerface protocol in a control plane between a UE and a base station,and FIG. 4 is a diagram exemplarily illustrating the structure of aradio interface protocol in a user plane between the UE and the basestation.

The radio interface protocol is based on the 3GPP wireless accessnetwork standard. The radio interface protocol horizontally includes aphysical layer, a data link layer, and a networking layer. The radiointerface protocol is divided into a user plane for transmission of datainformation and a control plane for delivering control signaling whichare arranged vertically.

The protocol layers may be classified into a first layer (L1), a secondlayer (L2), and a third layer (L3) based on the three sublayers of theopen system interconnection (OSI) model that is well known in thecommunication system.

Hereinafter, description will be given of a radio protocol in thecontrol plane shown in FIG. 3 and a radio protocol in the user planeshown in FIG. 4.

The physical layer, which is the first layer, provides an informationtransfer service using a physical channel. The physical channel layer isconnected to a medium access control (MAC) layer, which is a higherlayer of the physical layer, through a transport channel Data istransferred between the physical layer and the MAC layer through thetransport channel Transfer of data between different physical layers,i.e., a physical layer of a transmitter and a physical layer of areceiver is performed through the physical channel.

The physical channel consists of a plurality of subframes in the timedomain and a plurality of subcarriers in the frequency domain. Onesubframe consists of a plurality of OFDM symbols in the time domain anda plurality of subcarriers. One subframe consists of a plurality ofresource blocks. One resource block consists of a plurality of OFDMsymbols and a plurality of subcarriers. A Transmission Time Interval(TTI), a unit time for data transmission, is 1 ms, which corresponds toone subframe.

According to 3GPP LTE, the physical channels present in the physicallayers of the transmitter and the receiver may be divided into datachannels corresponding to Physical Downlink Shared Channel (PDSCH) andPhysical Uplink Shared Channel (PUSCH) and control channelscorresponding to Physical Downlink Control Channel (PDCCH), PhysicalControl Format Indicator Channel (PCFICH), Physical Hybrid-ARQ IndicatorChannel (PHICH) and Physical Uplink Control Channel (PUCCH).

The second layer includes various layers. First, the MAC layer in thesecond layer serves to map various logical channels to various transportchannels and also serves to map various logical channels to onetransport channel. The MAC layer is connected with an RLC layer, whichis a higher layer, through a logical channel. The logical channel isbroadly divided into a control channel for transmission of informationof the control plane and a traffic channel for transmission ofinformation of the user plane according to the types of transmittedinformation.

The radio link control (RLC) layer in the second layer serves to segmentand concatenate data received from a higher layer to adjust the size ofdata such that the size is suitable for a lower layer to transmit thedata in a radio interval.

The Packet Data Convergence Protocol (PDCP) layer in the second layerperforms a header compression function of reducing the size of an IPpacket header which has a relatively large size and contains unnecessarycontrol information, in order to efficiently transmit an IP packet suchas an IPv4 or IPv6 packet in a radio interval having a narrow bandwidth.In addition, in LTE, the PDCP layer also performs a security function,which consists of ciphering for preventing a third party from monitoringdata and integrity protection for preventing data manipulation by athird party.

The Radio Resource Control (RRC) layer, which is located at theuppermost part of the third layer, is defined only in the control plane,and serves to configure radio bearers (RBs) and control a logicalchannel, a transport channel, and a physical channel in relation toreconfiguration and release operations. The RB represents a serviceprovided by the second layer to ensure data transfer between a UE andthe E-UTRAN.

If an RRC connection is established between the RRC layer of the UE andthe RRC layer of a wireless network, the UE is in the RRC Connectedmode. Otherwise, the UE is in the RRC Idle mode.

Hereinafter, description will be given of the RRC state of the UE and anRRC connection method. The RRC state refers to a state in which the RRCof the UE is or is not logically connected with the RRC of the E-UTRAN.The RRC state of the UE having logical connection with the RRC of theE-UTRAN is referred to as an RRC_CONNECTED state. The RRC state of theUE which does not have logical connection with the RRC of the E-UTRAN isreferred to as an RRC_IDLE state. A UE in the RRC_CONNECTED state hasRRC connection, and thus the E-UTRAN may recognize presence of the UE ina cell unit. Accordingly, the UE may be efficiently controlled. On theother hand, the E-UTRAN cannot recognize presence of a UE which is inthe RRC_IDLE state. The UE in the RRC_IDLE state is managed by a corenetwork in a tracking area (TA) which is an area unit larger than thecell. That is, for the UE in the RRC_IDLE state, only presence orabsence of the UE is recognized in an area unit larger than the cell. Inorder for the UE in the RRC_IDLE state to be provided with a usualmobile communication service such as a voice service and a data service,the UE should transition to the RRC_CONNECTED state. A TA isdistinguished from another TA by a tracking area identity (TAI) thereof.A UE may configure the TAI through a tracking area code (TAC), which isinformation broadcast from a cell.

When the user initially turns on the UE, the UE searches for a propercell first. Then, the UE establishes RRC connection in the cell andregisters information thereabout in the core network. Thereafter, the UEstays in the RRC_IDLE state. When necessary, the UE staying in theRRC_IDLE state selects a cell (again) and checks system information orpaging information. This operation is called camping on a cell. Onlywhen the UE staying in the RRC_IDLE state needs to establish RRCconnection, does the UE establish RRC connection with the RRC layer ofthe E-UTRAN through the RRC connection procedure and transition to theRRC_CONNECTED state. The UE staying in the RRC_IDLE state needs toestablish RRC connection in many cases. For example, the cases mayinclude an attempt of a user to make a phone call, an attempt totransmit data, or transmission of a response message after reception ofa paging message from the E-UTRAN.

The non-access stratum (NAS) layer positioned over the RRC layerperforms functions such as session management and mobility management.

Hereinafter, the NAS layer shown in FIG. 3 will be described in detail.

The ESM (evolved Session Management) belonging to the NAS layer performsfunctions such as default bearer management and dedicated bearermanagement to control a UE to use a PS service from a network. The UE isassigned a default bearer resource by a specific packet data network(PDN) when the UE initially accesses the PDN. In this case, the networkallocates an available IP to the UE to allow the UE to use a dataservice. The network also allocates QoS of a default bearer to the UE.LTE supports two kinds of bearers. One bearer is a bearer havingcharacteristics of guaranteed bit rate (GBR) QoS for guaranteeing aspecific bandwidth for transmission and reception of data, and the otherbearer is a non-GBR bearer which has characteristics of best effort QoSwithout guaranteeing a bandwidth. The default bearer is assigned to anon-GBR bearer. The dedicated bearer may be assigned a bearer having QoScharacteristics of GBR or non-GBR.

A bearer allocated to the UE by the network is referred to as an evolvedpacket service (EPS) bearer. When the EPS bearer is allocated to the UE,the network assigns one ID. This ID is called an EPS bearer ID. One EPSbearer has QoS characteristics of a maximum bit rate (MBR) and/or aguaranteed bit rate (GBR).

FIG. 5 is a flowchart illustrating a random access procedure in 3GPPLTE.

The random access procedure is performed for a UE to obtain ULsynchronization with an eNB or to be assigned a UL radio resource.

The UE receives a root index and a physical random access channel(PRACH) configuration index from an eNodeB. Each cell has 64 candidaterandom access preambles defined by a Zadoff-Chu (ZC) sequence. The rootindex is a logical index used for the UE to generate 64 candidate randomaccess preambles.

Transmission of a random access preamble is limited to a specific timeand frequency resources for each cell. The PRACH configuration indexindicates a specific subframe and preamble format in which transmissionof the random access preamble is possible.

The UE transmits a randomly selected random access preamble to theeNodeB. The UE selects a random access preamble from among 64 candidaterandom access preambles and the UE selects a subframe corresponding tothe PRACH configuration index. The UE transmits the selected randomaccess preamble in the selected subframe.

Upon receiving the random access preamble, the eNodeB sends a randomaccess response (RAR) to the UE. The RAR is detected in two steps.First, the UE detects a PDCCH masked with a random access (RA)-RNTI. TheUE receives an RAR in a MAC (medium access control) PDU (protocol dataunit) on a PDSCH indicated by the detected PDCCH.

FIG. 6 illustrates a connection procedure in a radio resource control(RRC) layer.

As shown in FIG. 6, the RRC state is set according to whether or not RRCconnection is established. An RRC state indicates whether or not anentity of the RRC layer of a UE has logical connection with an entity ofthe RRC layer of an eNodeB. An RRC state in which the entity of the RRClayer of the UE is logically connected with the entity of the RRC layerof the eNodeB is called an RRC connected state. An RRC state in whichthe entity of the RRC layer of the UE is not logically connected withthe entity of the RRC layer of the eNodeB is called an RRC idle state.

A UE in the Connected state has RRC connection, and thus the E-UTRAN mayrecognize presence of the UE in a cell unit. Accordingly, the UE may beefficiently controlled. On the other hand, the E-UTRAN cannot recognizepresence of a UE which is in the idle state. The UE in the idle state ismanaged by the core network in a tracking area unit which is an areaunit larger than the cell. The tracking area is a unit of a set ofcells. That is, for the UE which is in the idle state, only presence orabsence of the UE is recognized in a larger area unit. In order for theUE in the idle state to be provided with a usual mobile communicationservice such as a voice service and a data service, the UE shouldtransition to the connected state.

When the user initially turns on the UE, the UE searches for a propercell first, and then stays in the idle state. Only when the UE stayingin the idle state needs to establish RRC connection, the UE establishesRRC connection with the RRC layer of the eNodeB through the RRCconnection procedure and then performs transition to the RRC connectedstate.

The UE staying in the idle state needs to establish RRC connection inmany cases. For example, the cases may include an attempt of a user tomake a phone call, an attempt to transmit data, or transmission of aresponse message after reception of a paging message from the E-UTRAN.

In order for the UE in the idle state to establish RRC connection withthe eNodeB, the RRC connection procedure needs to be performed asdescribed above. The RRC connection procedure is broadly divided intotransmission of an RRC connection request message from the UE to theeNodeB, transmission of an RRC connection setup message from the eNodeBto the UE, and transmission of an RRC connection setup complete messagefrom the UE to eNodeB, which are described in detail below withreference to FIG. 6.

1) When the UE in the idle state desires to establish RRC connection forreasons such as an attempt to make a call, a data transmission attempt,or a response of the eNodeB to paging, the UE transmits an RRCconnection request message to the eNodeB first.

2) Upon receiving the RRC connection request message from the UE, theENB accepts the RRC connection request of the UE when the radioresources are sufficient, and then transmits an RRC connection setupmessage, which is a response message, to the UE.

3) Upon receiving the RRC connection setup message, the UE transmits anRRC connection setup complete message to the eNodeB. Only when the UEsuccessfully transmits the RRC connection setup message, does the UEestablish RRC connection with the eNodeB and transition to the RRCconnected mode.

2. ProSe (Proximity Service)

As described above, ProSe service means a service that enables discoverybetween physically proximate devices and mutual direct communication,communication through a base station or communication through a thirddevice.

FIG. 7 illustrates a default data path through which two UEs performcommunication with each other in an EPS. This default data path passesthrough an eNodeB and a core network (i.e., EPC), which are managed byan operator. In the present invention, this path will be referred to asan infrastructure data path (or EPC path). Also, communication throughthis infrastructure data path will be referred to as infrastructurecommunication.

FIG. 8 illustrates a direct mode data path between two UEs based onProSe. This direct mode communication path does not pass through theeNodeB and the core network (i.e., EPC), which are managed by anoperator. FIG. 8(a) illustrates a case that UE-1 and UE-2 are camping ondifferent eNodeBs and exchange data through a direct mode communicationpath. FIG. 8(b) illustrates a case that two UEs are camping on the sameeNodeB and exchange data through a direct mode communication path.

FIG. 9 illustrates a locally routed data path through eNodeB between twoUEs based on ProSe. This communication path through eNodeB does not passthrough a core network (i.e., EPC) managed by an operator.

A non-roaming reference architecture is shown in FIG. 10. In thestructure of FIG. 10, the EPC may determine proximity of two UEs andperform an EPC-level ProSe discovery procedure to notify the UE of thedetermined result. For this EPC-level ProSe discovery, a ProSe Functionserves to determine proximity of two UEs and notify the UE of thedetermined result.

The ProSe function may retrieve and store ProSe associated subscriberdata and/or ProSe associated subscriber data from HSS, and performauthentication and configuration for EPC level ProSe discovery and EPCsub WLAN direct discovery communication. Also, the ProSe function may beoperated as a location service client that enables EPC level discovery,and may provide the UE of information for assisting WLAN directdiscovery and communication. The ProSe function handles EPC ProSe UserIDs and Application Layer User ID, and exchanges a signal with a thirdparty application server for application registration identifiermapping. For transmission of a proximity request, proximity alerts andlocation report, the ProSe function exchanges a signal with a ProSefunction of other PLMNs. In addition, the ProSe function providesvarious parameters required for ProSe discovery and ProSe communication.Details of the ProSe function are based on 3GPP TS 23.303.

FIG. 11 illustrates communication through a ProSe UE-to-Network Relay.When a remote UE has connectivity to an EPC through a UE-to-networkrelay, the remote UE can communicate with an application server (AS) orparticipate in group communication. FIG. 12 shows an example in which aremote UE participate in group communication. UE-1 to UE-6 which are UEsbelonging to the same group in FIG. 12 may receive downlink trafficthrough unicast or MBMS for specific media that configure groupcommunication. As a result, although not in E-UTRAN coverage, the remoteUE may transmit media traffic to other group members (that is, generateuplink traffic) by joining group communication through the UE-to-NetworkRelay or receive media traffic transmitted from other group members. InFIG. 12, a GCS AS (Group Communication Service Application Server) mayserve to i) exchange GC1 signalling, ii) receive uplink data from aunicast UE, iii) transfer data to all UEs, which belong to a group, byusing Unicast/MBMS delivery, iv) transmit application level sessioninformation through Rx interface to a PCRF, and v) support a servicecontinuity procedure for a UE which is switched between Unicast Deliveryand MBMS Delivery. The GCS AS, Public Safety AS, and GCSE AS (GroupCommunication Service Enabler Application Server) may be interpreted torefer to the same meaning and include AS that controls/managescommunication joined by a plurality of UEs. Details of groupcommunication is based on TS 23.468.

FIG. 13 illustrates a procedure in which a remote UE that is not servedby an E-UTRAN performs direct communication through a UE-to-networkrelay. A UE capable of operating as a ProSe UE-to-network relay mayestablish a PDN connection to provide relay traffic to the remote UE byaccessing the network. The PDN connection supporting the UE-to-networkrelay is used only to provide the relay traffic to the remote UE.

First, a relay UE establishes a PDN connection through initial access toan E-UTRAN [S1310]. In the case of IPv6, the relay UE obtains an IPv6prefix using a prefix delegation function. Next, the relay UE performs adiscovery procedure, which differs depending on either Model A or ModelB, together with a relay UE [S1320]. The remote UE selects the relay UEdiscovered through the discovery procedure and then establishesone-to-one direct connection [S1330]. If there is no PDN connectionassociated with a relay UE ID or if an additional PDN connection forrelay operation is required, the relay UE initiates a new PDN connectionprocedure [S1340].

Next, an IPv6 prefix or an IPv4 address is allocated to the remote UE[S1350], and then uplink/downlink relay operation is initiated. When theIPv6 prefix is allocated, an IPv6 stateless address auto-configurationprocedure configured with router solicitation signaling from the remoteUE to the relay UE and router advertisement signaling from the relay UEto the remote UE is initiated. On the other hand, when the IPv4 addressis allocated, an IPv4 address allocation using DHCPv4 procedureconfigured with DHCPv4 discovery signaling (from the remote UE to therelay UE), DHCPv4 offer signaling (from the relay UE to the remote UE),DHCPv4 request signaling (from the remote UE to the relay UE), andDHCPv4 ACK signaling (from the relay UE to the remote UE) is initiated.

Thereafter, the relay UE performs a Remote UE Report procedure forinforming an MME that the relay UE is connected to the remote UE[S1360]. The MME performs a Remote UE Report Notification procedure toinform an SGW and a PGW that the new remote UE is connected [S1370].Then, the remote UE performs communication with the network through therelay UE [S1380]. Details of the direct connection generation procedurecould be found in TS 23.303.

3. Tracking Area Update (TAU) Procedure

FIG. 14 illustrates a case in which an SGW is changed together with aTAU procedure. In FIG. 14, an MME is also changed. When the TAUprocedure is triggered, a UE transmits a TAU request message to anevolved node B (eNodeB or eNB). In this case, while a procedure forestablishing a NAS signaling connection (e.g., attach/TAU/servicerequest) is performed, a NAS layer of the UE provides a lower layer(e.g., AS layer) with an S-TMSI (System Architecture Evolution (SAE)Temporary Mobile Subscriber Identity), which is allocated by a previousMME, or a GUMMEI (Globally Unique MME Identifier) of the previous MME.When the tracking area (TA) of the current cell is in the list of TAsthat the UE previously registered in the MME during the NAS signallingconnection establishment, the NAS layer of the UE provides the lowerlayer with the S-TMSI rather than the MME identifier. On the contrary,when the TA of the current cell is not in the list of TAs that the UEpreviously registered in the MME during the NAS signalling connectionestablishment, the NAS layer of the UE provides the lower layer with theMME identifier part of the GUTI with an indication that the MMEidentifier is a native GUMMEI. (when the tracking area of the currentcell is in the list of tracking areas that the UE previously registeredin the MME during the NAS signalling connection establishment, the UENAS shall provide the lower layers with the S-TMSI, but shall notprovide the registered MME identifier to the lower layers; or When thetracking area of the current cell is not in the list of tracking areasthat the UE previously registered in the MME during the NAS signallingconnection establishment, the UE NAS shall provide the lower layers withthe MME identifier part of the valid GUTI with an indication that theidentifier is a native GUMMEI).

The above-described S-TMSI or GUMMEI is provided to the AS layertogether with the TAU request message, and the AS layer of the UEforwards the received S-TMSI or GUMMEI to the eNB. The eNB performs anNNSF (NAS Node Selection Function) based on the received S-TMSI orGUMMEI and then selects an MME that will receive the received NASmessage (e.g., TAU request message). In this case, the eNB may selectthe MME through an MMEC (MME Group Identifier) included in the S-TMSI oran MMEG (MME Group Identifier) and an MMEC (MME code) in the GUMMEI.When the S-TMSI is provided, the previous MME is selected by the MMEC.In addition, the eNB provides the S-TMSI to the MME, and thus, the MMEcan identify the UE through the S-TMSI.

On the contrary, when the GUMMEI is provided, the NNSF is performedbased on the GUMMEI, and then the MME is selected. In this case, the MMEmay or may not be changed. When a new MME is selected, the new MMEderives the GUMMEI of the previous MME from the GUTI in ‘old GUTI IE’included in the TAU request message and checks whether there is aninterface with the corresponding MME. If signaling with thecorresponding MME is possible, the TAU request message including theGUMMEI, which is the identifier of the previous MME, is sent to the newMME, and the new MME requests information on the UE by transmitting acontext request message to the previous MME. After receiving the contextinformation on the UE through a context response message from theprevious MME, the new MME performs authentication and securityprocedures for the UE with respect to an HSS. When the new MME transmitsa context acknowledgement message to the previous MME, an MME changeprocedure is completed.

Next, the new MME transmits a create session request message to a newSGW, which is changed (i.e., relocated). When the RAT (Radio AccessTechnology) is changed due to the MME change, the new MME transmits amodify bearer request message to a PGW, and RAT information istransmitted from the PGW to a PCRF. The PGW updates a bearer context andtransmits a modify bearer response message to the SGW. If the update ofthe bearer context is completed, the SGW transmits a create sessionresponse message to the new MME.

When the new MME does not have subscriber information of the UE or in ascenario of sharing a specific network, the new MME transmits an updatelocation message to the HSS. After receiving the message, the HSSexchanges a cancel location message and a cancel locationacknowledgement message with the previous MME. The previous MME deletesthe context information of the UE. In the case of an SGSN, the SGSNexchanges an Iu release command message and an Iu release completemessage with an RNC.

The HSS transmits an update location acknowledge message to the new MMEas a response. Meanwhile, the previous MME/SGSN transmits a deletesession request message to the previous SGW to release EPS bearerresources. The previous SGW transmits a delete session response messageand deletes packets stored in the buffer.

Meanwhile, when the UE cannot access the corresponding TA due toregional subscription restriction or access restriction, the new MMEtransmits a TAU reject message to the UE. Otherwise, the new MMEtransmits a TAU accept message to the UE. In this case, the TAU acceptmessage includes a GUTI (Globally Unique Temporary Identifier) of thenew MME. After receiving the TAU accept message, the UE transmits a TAUcomplete message. By doing so, the TAU procedure is completed.Meanwhile, the details of the TAU procedure and the definitions of theTAU request message, TAU reject message, TAU accept message, and TAUcomplete message including information included therein are based on TS24.301.

4. Proposed D2D Direct Communication Method

In the case of an existing UE-to-network relay UE, since theUE-to-network relay UE operates on the assumption that a remote UE isout of coverage, a network is not interested in managing a context ofthe remote UE. If context information of the remote UE is stored in thenetwork, unnecessary paging may occur.

However, the proposed embodiment considers a case in which not only therelay UE but also the remote UE are in coverage and proposes that thenetwork recognizes the remote UE and manage the context of the remoteUE. When the network manages the context information of the remote UE, aprocedure in which the relay UE allocates an IP address for the remoteUE may be omitted.

Recently, beyond the smartphone or mobile terminal, various IoT deviceshave been introduced, and as a representative example, a wearabledevice, a smartwatch, a vehicle terminal, etc. are considered. Suchterminals (UEs) can be implemented to communicate with the network in astand-alone manner. In the case of a UE capable of stand-alonecommunication with the network, signaling with the network occurs ineach UE. That is, since as the number of IoT UEs increases, the amountof signaling exponentially increases, the network may have a serioussignaling overhead problem.

Meanwhile, low-power UEs can be divided into the following three types:i) a UE that is independent and has a cellular identity; ii) a UE thatis dependent and has a cellular identity; and iii) a UE that isdependent and has no cellular identity. In the proposed embodiment, ascenario in which among the three types of UEs, the second type of UE,specifically, an IoT UE that is dependent and has a cellular identitycoexists with an independent UE is considered. In particular, methodsperformed by a network for efficiently managing UEs and subscriberinformation of the corresponding UEs will be described.

FIGS. 15 to 17 illustrate direct communication methods according toproposed embodiments.

Although a user may use various UEs independently, the user may use atleast one UE at the same time. For example, a mobile UE and an IoT UEmay be simultaneously used, or a plurality of mobile UEs and IoT UEs maybe simultaneously used. In the proposed embodiment, when a plurality ofUEs coexist, a method for reducing power consumption andmobility-related signaling of the UEs by establishing a relationshipamong the UEs and controlling signaling will be described. In addition,it is assumed in the following description that a UE has a capability ofindependently communicating with an EPC.

Hereinafter, a description will be given of a process for establishing arelationship among a plurality of UEs. The relationship establishmentprocess can be divided into two methods. First, according to apreconfigured method, the network has a plurality of pieces of user (orsubscriber) information for individual UEs. In addition, the networkrequires additional information to recognize a relationship among theindividual pieces of user (or subscriber) information, and each UE alsorequires additional information to recognize the relationship among theindividual pieces of user information. Thereafter, the network andindividual UEs establish the relationship using the above-describedadditional information. Details will be described later.

The additional information may include at least one of information on aUE group and information on classes or priorities of UEs. In this case,the information on the UE group may be stored as a group identifierindicating a specific group or it may include identifiers of UEs. Theinformation on the classes or priorities of the UEs may mean informationindicating relative classes or priorities among the UEs when one or moreUEs exist, and it may mean information on a subordinate relationship(e.g., primary/secondary, master/slave, etc.). The network andindividual UEs may establish the relationship among the plurality of UEsusing the above-described additional information together with theindividual user information (or subscriber information).

Second, according to a non-preconfigured method, even when there is nopreconfigured information for recognizing a relationship between UEs,the relationship may be configured through mutual agreement among theUEs. The UEs may perform the agreement procedure by informing thenetwork of their intentions. Details will be described later.

Next, based on the above-described two methods, particular processes forestablishing the relationship among the plurality of UEs will bedescribed. If the UEs are close to each other, the UEs perform a D2Ddiscovery procedure. In this case, the D2D connection may be establishedthrough E-UTRAN corresponding to the 3GPP network, Wi-Fi Direct, orBluetooth.

While the adjacent UEs establish the connection, a representative UE isconfigured. The process for configuring the representative UE may bedifferently performed according to the above-described two relationshipestablishment methods. In the preconfigured method, the representativeUE is selected in advance from among the plurality of UEs. That is, whenthe additional information for recognizing the relationship among theUEs includes the information on the classes or priorities of the UEs,not only the network but also the UEs have the additional information.Thus, the representative UE can be determined in advance from among theplurality of UEs according to the corresponding class or priority.Meanwhile, in the following description, ‘representative UE’ means atleast one UE that represents a plurality of UE groups, and ‘subordinaryUEs’ may mean other UEs except the representative UE in the plurality ofUE groups. The UE group may be composed of at least one representativeUE and at least one subordinary UE.

Second, in the non-preconfigured method, the representative UE may bedetermined by signaling among the UEs. Alternatively, it may bedetermined by the network through signaling from the network. In thiscase, signaling may include information on capability or power of theUEs as well as the above-described class or priority information of theindividual UEs. Specifically, the power information may represent theremaining battery amount or power efficiency of each UE. That is, theinformation for selecting the representative UE from among the pluralityof UEs may be transmitted and received among the UEs or to the networkin order to configure the representative UE.

Specifically, the UEs may transmit and receive the above-mentionedinformation using a PC5 message through a PC5 interface to configure therepresentative UE. However, if the individual UEs request the network toselect the representative UE, the network may check requests from theUEs and then select the representative UE from among the plurality ofUEs. In this case, there may be a separate application server in chargeof establishing the relationship among the UEs and managing theinformation for selecting the representative UE. The network requeststhe application server to establish the relationship among the UEs andthen receive the corresponding results, and the application server maybe considered as a function or functional entity for establishing therelationship and determining the representative UE.

If the representative UE is configured, each UE activates a UE groupincluding a plurality of UEs. In other words, a prescribed UE groupincluding a plurality of UEs is formed. Each UE transmits signaling tothe network (e.g., MME) to activate the UE group. For example, if aspecific UE transmits a request for activating the UE group through NASsignaling (for example, a TAU request message or a newly defined NASmessage) to the MME, the MME transmits a response to the request throughdifferent NAS signaling (for example, a TAU accept message, a TAU rejectmessage, or a newly defined NAS message) to the UE. The request messagefor activating the UE group may include at least one of an identifier ofthe UE group, identifiers of UEs belonging to the UE group, and anindicator/information element (IE) for requesting the activation of theUE group.

The configured representative UE may transmit the aforementioned requestfor the UE group activation to the network. Alternatively, all of theplurality of UEs may transmit the request for the UE group activation tothe network, respectively. In the latter case, after collecting therequests from the UEs, the network may respond to all the UEs or onlythe representative UE. In addition, upon receiving the request for theUE group activation from one or more UEs, the network may performinteraction with the application server in charge of establishing therelationship among the UEs if necessary.

Hereinafter, a description will be given of how UEs and network entitiesoperate after the UE group activation. When the relationship among theUEs is configured, the UE group is formed, and then the representativeUE is selected as described above, the UEs and network entities (e.g.,MME) recognize the relationship among the plurality of UEs. Based onaccess schemes for D2D communication, the operation of subordinary UEscan be divided into the following cases: i) when D2D communication isperformed through the E-UTRAN; and ii) when D2D communication isperformed through Wi-Fi or Bluetooth rather than the E-UTRAN. In theformer case, each of the plurality of UEs performs sidelink operationfor D2D communication only and deactivates AS layer operation for an Uuinterface with the EPC. That is, the eNodeB (eNB) may allocate radioresources for the sidelink operation to the plurality of UEs. On theother hand, in the latter case, each of the plurality of UEs maydeactivate all AS layers. That is, since there is no process where theeNodeB (eNB) allocates sidelink radio resources, the plurality of UEs inthe latter case may deactivate sidelinks as well.

In the following description, the definition of the AS layer operationmay vary depending on D2D connection types. When a D2D connection isestablished through the E-UTRAN as described above, the AS layeroperation may mean communication operation with the EPC through the Uuinterface. On the other hand, when a D2D connection is establishedthrough Wi-Fi or Bluetooth rather than the E-UTRAN, the AS layeroperation may mean both the communication operation with the EPC throughthe Uu interface and the sidelink operation.

Hereinafter, a description will be given of a case in which mobileterminating (MT) call/data is received. When receiving downlink datanotification (DDN) for a specific UE, a network entity (e.g., MME)recognizes a UE group to which the UE corresponding to a DDN targetbelongs and performs paging of a representative UE of the UE group towhich the corresponding UE belongs. A paging message transmitted to therepresentative UE may include an identifier of the specific UEcorresponding to the DDN target.

After receiving the paging message, the representative UE of the UEgroup forwards the paging message to the specific UE corresponding tothe target through a sidelink. After receiving the paging message, theUE activates the AS layer and performs cell selection. If the UEcompletes the cell selection process successfully, the UE performs aservice request (SR) procedure, receives the MT call/data, and transmitsa paging response message to the network entity. If the UE completes thereception of the MT call/data, the UE deactivates the AS layer again andperforms operation for the sidelink communication with therepresentative UE of the UE group. The UE may deactivate the AS layerimmediately after completing the reception of the MT call/data or afterentering an EMM-IDLE state.

Meanwhile, during the above-described processes, the representative UEof the UE group may monitor one paging message or a plurality of pagingmessages at a time. In the former case, the paging message isimplemented to include the identifier of the UE belonging to the UEgroup. In the latter case, the representative UE of the UE group maymonitor paging messages for a plurality of UEs belonging to the UE groupat a time.

Next, a description will be given of a case in which mobile originating(MO) call/data is transmitted. When there is MO call/data or MOsignaling from a UE, the UE activates the AS layer and performs the cellselection. If the UE completes the cell selection successfully, the UEperforms the SR procedure and transmits the MO call/data or MO signalingto the network. If the UE completes the transmission of the MO call/dataor MO signaling, the UE deactivates the AS layer again and performs theoperation for the sidelink communication with the representative UE.Similar to the MT call/data, the UE may deactivate the AS layerimmediately after transmitting the MO call/data or after entering theEMM-IDLE state.

Hereinafter, the deactivation of the UE group will be explained. The UEgroup, which was activated according to the aforementioned embodiment,can be deactivated in any one of the following cases, for example: whenthe quality of a sidelink connection between each subordinary UE and therepresentative UE in the UE group is lower than a predeterminedthreshold; when a sidelink connection is disconnected; and when a UEbelonging to the UE group desires to deactivate the UE group. In thiscase, any UE belonging to the UE group or the representative UE maytransmit NAS signaling (for example, a TAU message used in the TAUprocedure or a newly defined NAS message) to a network entity todeactivate the UE group. In this case, the NAS signaling message mayinclude an indicator or information element (IE) indicating thedeactivation of the UE group. When the UE group is deactivated, thenetwork entity and a plurality of UEs can communicate with the EPC in astand-alone manner, i.e., in the same manner as before the UE group isformed.

Hereinabove, a series of processes in which a plurality of UEs form a UEgroup based on a predetermined relationship, communicate using the UEgroup, and deactivate the UE group have been described. The abovedescription has been made on the assumption that one MME has contextinformation of a plurality of UEs. Hereinafter, a description will begiven of an operation method when individual UEs belong to differentMMEs in the situation that a UE group is formed.

First, a process for PLMN alignment between a relay UE and a remote UEwill be described. A UE that intends to form a UE group and arepresentative UE should select and register the same PLMN. In thiscase, the relay UE may be the representative UE, and the remote UE maybe a subordinary UE belonging to the UE group. Here, a PLMN of therepresentative UE (relay UE) can be aligned with respect to (changed to)a PLMN of the subordinary UE (remote UE) and vice versa.

The UEs share information on the currently registered PLMNs (RPLMNs)with each other. To this end, PLMN IDs of the RPLMNs are included in PC5messages, and then the messages can be transmitted and received.Alternatively, an identifier including a PLMN ID (e.g., GUTI) isincluded in a PC5 message, and then the PC5 message can be transmittedand received. In the latter case, after receiving the GUTI, a UE canextract the PLMN ID from the GUTI. Meanwhile, as the PC5 message forsharing the PLMN information, a PC5 discovery message (e.g., PC5discovery announcement message, PC5 discovery solicitation message, PC5discovery response message, etc.) can be used. Alternatively, PC5signaling message (e.g., direct communication request message or directcommunication accept message used in a direct communicationestablishment procedure) can be used.

After the relay UE and remote UE share the PLMN information, any one ofthem performs a PLMN selection procedure to register in the PLMN of theother UE. As the first method, according to a conventional PLMNselection procedure, a ProSe layer of the specific UE transmitsinformation on a register PLMN of the other UE to the NAS layer, the NASlayer of the UE transmits, to the AS layer, an indicator indicating thatPLMN selection is required, and then the AS layer transmits broadcastedPLMN IDs to the NAS layer by searching E-UTRAN bands. At this time, ifthe register PLMN of the other UE is included in the PLMN list, the NASlayer may check that and select the register PLMN. As the second method,the ProSe layer of the specific UE transmits the PLMN information of theother UE to the NAS layer, and the NAS layer transmits to the AS layerthat the PLMN selection process for the register PLMN of the other UE isrequired together with the corresponding PLMN ID. The AS layer checkswhether the register PLMN is included in the broadcasted PLMN IDs. Whenthe corresponding PLMN ID is included in the broadcasted PLMN IDs, theAS layer transmits, to the NAS layer, an indicator indicating that thecorresponding PLMN ID is included. Thereafter, the NAS layer may selectthe corresponding PLMN. As the third method, it may be considered thatthe specific UE sets its PLMN as the register PLMN provided by the otherUE.

When the same PLMN is selected by the remote UE and relay UE asdescribed above, the remote UE does not perform the PLMN selectionprocedure anymore before releasing the direct connection with the otherUE (relay UE) to avoid unnecessary PLMN switching. When the PLMNalignment process is successfully completed, the remote UE performs anMME alignment (change) process. In this case, after the completion ofthe PLMN alignment process, a cell alignment (change) process may beperformed before the MME alignment (change) process. To this end, theremote UE and relay UE share cell IDs of the cell which they arecurrently camped on using PC5 messages. When the cell corresponding tothe received cell ID is selected, the remote UE and relay UE mayinitiate the cell alignment process.

In addition, MME information may be included in a PC5 message, which istransmitted and received while each of a plurality of UEs performs a D2Ddiscovery procedure or a direct link setup. That is, when the PC5message is transmitted and received, the UEs may share their MMEinformation with each other. The MME information included in the PC5message may be a UE's GUTI or an MME's GUMMEI.

When the UEs share their MME information as described above, the UEs mayrecognize that they belong to different MMEs (i.e., they are supportedby different MMEs). In this case, the UEs belonging to the differentMMEs may operate as follows. First, a plurality of UEs belonging todifferent MMEs may operate as they belong to the same MME. Second, whilea plurality of UEs still belong to different UEs, a representative UEmay forward DDN to other UEs in a UE group. Hereinafter, the firstmethod will be described.

Specifically, a certain UE belonging to the UE group performs the TAUprocedure for exchanging its MME. In this case, except therepresentative UE, any subordinary UE in the UE group may become the UEthat intends to change the MME. In addition, the representative UE maybe a relay UE, and the subordinary UE belonging to the UE group may be aremote UE. Moreover, an MME of the subordinary UE (remote UE) may bechanged to that of the representative UE (relay UE) and vice versa. Theoperation of changing an MME can be achieved when the representative UEor subordinary UE performs the TAU procedure to a target MME or when thesubordinary UE performs the TAU procedure to the currently connectedMME.

When the TAU procedure to the target MME is performed, the target MMEmay bring context information of a UE from a previous MME as in theprior art. In this case, identification information of the target MMEmay be included in an RRC message or TAU request message to exactlyrepresent the target MME. When the RRC message contains theidentification information of the target MME, an eNB transmits the TAUrequest message to the target MME. Such a process will be described indetail with reference to FIGS. 15 and 16.

Meanwhile, when the TAU procedure to the currently connected MME isperformed, an MME that receives the TAU request message including theidentification information of the target MME (i.e., an MME other thanthe target MME, for example, an MME currently connected to the UE)checks the identification information of the target UE included in theTAU request message and then forwards the TAU request message to thetarget MME. This forwarding process may be performed through redirectionby the eNB. To bring the context information of the UE from the previousMME, the target MME may perform the TAU procedure by receiving the TAUrequest message from another MME. It will be described in detail withreference to FIG. 17. In this case, after receiving the TAU requestmessage, the connected MME may not forward the received TAU requestmessage to the target MME. For example, when the eNB cannot perform theredirection due to no interface with the target MME even though theconnected MME requests the eNB to perform the redirection, the eNBinforms the connected MME of this fact. Thereafter, the MME connected tothe eNB may transmit a NAS message (e.g., TAU reject message) to the UEby including a cause why the redirection cannot be performed in the NASmessage. By doing so, the remote UE can perform the TAU procedure to thetarget MME or perform an attach procedure.

Meanwhile, when the TAU procedure to the target MME is performed, if theidentification information of the target MME is included in the TAUrequest message, the target MME may receive the TAU request message. Inthis case, if the target MME is able to perform interaction with theprevious MME, there may be no problem. However, in some cases, it maynot operate. For example, when the eNB fails to discover the target MME,when the TAU request message cannot be transmitted due to no interfacewith the target MME, when the target UE cannot discover the previous MMEeven though it receives the TAU request message, or when the interactionis impossible due to no interface, there may be problems. In each errorcase, the eNB and target MME need to transmit, to the UE, a cause whythe corresponding error case occurs by including the cause in the RRCmessage or NAS message (e.g., TAU reject message including the rejectcause). Thus, the UE may perform an additional process for the TAUprocedure to the currently connected MME or the attach procedure to thetarget MME.

When the target MME bring the context information of the UE from theprevious MME, different MMEs of UEs may be set as the same MME. Thus, anMME that receives DDN from an SGW may operate in a similar way to the UEgroup activation process for the same MME.

Hereinabove, the case in which a plurality of UEs belonging to anactivated UE group have different MMEs has been described. On the otherhand, when a UE group is activated, if UEs belonging to the UE grouphave different MMEs, the UEs may be configured to have the same MME.That is, when the UE group is activated, if UEs in the UE group havedifferent MMEs, the UEs may be configured to belong to the same MME(e.g., a representative UE's MME). Such a process can be implemented byallowing the representative UE's MME to receive context information ofUEs from other UEs' MMEs, or it can be implemented by allowing otherUEs' MMEs rather than the representative UE's MME to transmit contextinformation of UEs to the representative UE's MME. In this case, theGUTI or GUMMEI can be exchanged as described above to check whether MMEsare identical to each other.

In addition, if the UEs are configured to belong to the same MME whilethe UE group is activated, the corresponding MME may inform the MMEchange through interaction with an SGW and PGW. For example, it isassumed that when UE 1 is served by MME 1 and a PDN connection thereofis established through SGW 1 and PGW 1, and when UE 2 is served by MME 2and a PDN connection thereof is established through SGW 2 and PGW 2, UE1 is a representative UE. In this case, the serving MME of UE 2 isswitched from MME 2 to MME 1, and MME 1 may inform SGW 2 and PGW 2 thatthe serving MME of UE 2 is changed to MME 1.

Next, the second method (i.e., paging redirection) where while aplurality of UEs still belong to different UEs, a representative UEforwards DDN to other UEs in a UE group will be described.

After a plurality of UEs recognize that they belong to different MMEs,DDN may be transmitted to an MME if MT call/data of a UE except arepresentative UE occurs. In this case, after receiving the DDN, the MMEmay forward the received DDN to an MME to which the representative UEbelong.

Specifically, a subordinary UE transmits NAS signaling (e.g., a messageused in the TAU procedure or new NAS signaling message), and thus an MME(i.e., previous MME) to which the corresponding UE belongs informs theMME to which the representative UE belongs (i.e., target MME) that aspecific UE in the UE group belongs to a different MME. By doing so,both the previous MME and target MME recognize that the specific UE'sMME is different from the representative UE's MME. In this case, amessage transmitted to the target MME may include an identifier of a UEor an identifier of the UE group. In addition, the message may furtherinclude an indicator for paging forwarding (or paging redirection). Whenan MME of another UE rather than the representative UE receives the DDNof the corresponding UE, the MME forwards the DDN message to the targetMME. In this case, the forwarded DDN message may include an indicatorindicating paging forwarding and an identifier of the corresponding UE.After receiving the DDN message, the target MME performs a pagingprocedure and includes the identifier of the corresponding UE (i.e.,another UE rather than the representative UE) in a paging message.

Meanwhile, after receiving the paging message in accordance with theDDN, the representative UE transmits the paging message or a PC5 messageincluding information indicating that the paging message was received toa UE corresponding to the paging target. After receiving the PC5message, the UE performs a service request procedure to the currentlyregistered MME. Upon receiving the service request message transmittedfrom the UE corresponding to the paging target, the MME transmits asignaling message to the target MME to inform that the service requestmessage is received. The target MME receiving the signaling messagerecognizes that the paging message transmission is successfullyperformed and then completes the paging message transmission procedure.Meanwhile, when receiving the service request message, the MME performsthe service request procedure as in the prior art.

Next, a description will be given of mobility management after the UEgroup activation (i.e., a case in which services are provided by a newMME due to movements of UEs belonging to the UE group). First, whenindividual UEs' context information is stored in the same single MME asdescribed above, if mobility occurs, only the representative UE amongthe UEs in the UE group performs the TAU procedure. That is, when therepresentative UE is in charge of the mobility management because the UEgroup is activated, a subordinary UE does not separately perform the TAUprocedure in accordance with mobility even when it is in the coverage.Meanwhile, when the TAU procedure needs to be performed for otherpurposes such as capability or parameter update, a subordinary UE mayrequest the representative UE to perform the TAU procedure through a PC5message, and thus the representative UE may perform the TAU procedure.Alternatively, the subordinary UE may perform the TAU procedure.

The TAU request message transmitted by the representative UE may includethe identifier of the UE group or identifiers of UEs belonging to the UEgroup. When the new MME intends to bring context information from theprevious MME after receiving the TAU request message from therepresentative UE, the new MME send a request by including theidentifier of the UE group, the identifiers of the UEs belonging to theUE group, or a prescribed indicator (for example, an indicator forrequesting contexts of all the UEs belonging to the UE group). Whenreceiving the request, the previous MME transmits context informationrelated to all the UEs (i.e., representative UE and subordinary UEs)belonging to the UE group to the new MME. In this case, contextinformation of a subordinary UE (e.g., remote UE) may include a user IDof the UE and information on the allocated IP. When the TA of thecurrent cell is not included in the list of TAs previously registered bythe UE during the NAS signalling connection establishment, the TAUrequest message transmitted by the representative UE may include theidentifier of the UE group or the identifiers of the UEs belonging tothe UE group.

When the individual UEs' context information is stored in differentMMEs, the representative UE in the UE group performs the TAU procedure.In this case, the TAU request message may include the identifier of theUE group, the identifiers of the UEs belonging to the UE group, or aprescribed indicator (for example, an indicator for requesting contextinformation of all the UEs belonging to the UE group). The new MMEperforms a context request procedure to an MME supporting the UEsbelonging to the UE group. If there are more than two MMEs supportingthe UEs, the new MME performs the context request procedure to each ofthe MMEs. By doing so, each of the MMEs transmits its UE contextinformation to the new MME. If an MME of a specific UE rather than therepresentative UE of the UE group needs to handle context information,the MME in charge of the context information may be changed through aredirection procedure performed by an eNB or a direct redirectionprocedure between MMEs.

Next, a GUTI allocation procedure after the MME change will beexplained. When MMEs of UEs belonging to the UE group are changedaccording to the above-described processes, each MME needs to allocate anew GUTI and transmits the GUTI to each UE. It may be transmittedthrough a TAU accept message or a newly defined different message. Therepresentative UE needs to be allocated a plurality of GUTIs includingGUTIs of other UEs belonging to the UE group, and the new MME maytransmit, to the representative UE, a single TAU accept message byincluding all GUTIs of a plurality of UEs. The message carrying theallocated GUTIs includes not only each UE's identifier but also amapping relationship between each UE's identifier and each of theallocated GUTIs so that when receiving this message, the representativeUE can recognize each UE's GUTI. If MMEs of a plurality of UEs arechanged to the same MME, a GUTI may be allocated to a UE without the MMEidentifier. In other words, an M-TMSI corresponding to each UE'sidentifier may be transmitted. After receiving a plurality of GUTIs, therepresentative UE transmits the allocated GUTIs to the UEs throughsidelinks.

On the other hand, the new MME may transmit, to each UE, the TAU acceptmessage for allocating the GUTI. Since the message (e.g., TAU acceptmessage) carrying each allocated GUTI includes the identifier of acorresponding UE, the representative UE can recognize each UE's GUTIupon receiving the message. In this case, after receiving a plurality ofTAU accept messages, the representative UE transmits newly allocatedGUTIs to the individual UEs through sidelinks. Similarly, when MMEs of aplurality of UEs are changed to the same MME, an MME identifier may beexcluded from a GUTI.

Hereinafter, the process in which a plurality of UEs form a UE group andthe process in which a representative UE is selected will be describedagain with reference to a ProSe communication process. That is, a casewhere when a relay UE and a remote UE, which are close to each other,form a UE group, the two UEs belong to different MMEs will be described.In this case, if the two UEs are located at the same position, they maybelong to different MMEs. This is because an eNB can form an interfacewith at least one MME and provides a UE with an MME suitable for the UEthrough an MME selection function when the UE access the eNB. Anotherreason is that a dedicated core network (DCN), which includes an MME,SGW, and PGW according to usage type of the UE may be changed based onDCN functionality, and thus, a different DCN may be registered dependingon UEs. Further, it is because when the location of a UE is not changed,the UE may belong to a different MME due to load balancing.

The MME alignment process will be described in detail with reference toFIGS. 15 to 17. The MME alignment process, which will be describe later,can be performed with the UE group activation process. For convenienceof description, the following assumptions are applied: a relay UE is arepresentative UE and a remote UE is a UE belonging to a UE group; theremote UE is aligned to an MME to which the relay UE belongs; and therelay UE is served by MME 1 and remote UE is served by MME 2.

FIG. 15 illustrates the MME alignment process initiated by the relay UE.First, the remote UE transmits, to the relay UE, a TAU request messagein the form of a PC5 message [S1505]. This PC5 message may correspond toa PC5 signaling message (e.g., a direct communication request message ordirect communication accept message used in a direct connectionestablishment procedure), and it may include an indicator indicatingthat MME alignment is required. In addition, the TAU request message ofthe remote UE may be transmitted as itself, or it may be transmitted asa specific IE, parameter, or indicator indicating a TAU request, whichis included in the TAU request message. Moreover, the TAU requestmessage includes a GUTI of the remote UE.

After receiving the PC5 message, the relay UE triggers a TAU procedurefor the remote UE [S1510]. The relay UE forwards the TAU request messagereceived from the remote UE to MME 1, which corresponds to theregistered MME [S1515 c and S1520]. When the TAU request message of theremote UE is expressed as an IE, parameter, or indicator as describedabove, the relay UE directly configures the TAU request message of theremote UE and then transmit it to an eNB and MME 1 in step S1520.Specifically, when a TA of the current cell is included in the list ofTAs registered by a UE, a NAS layer of the UE transmits an S-TMSI to anAS layer. When the AS layer is provided with the S-TMSI by the NASlayer, the AS layer transmits the S-TMSI to the eNB by including it inRA msg3 [S1515 c]. On the other hand, when the TA of the current cell isnot included in the list of TAs registered by the UE, the NAS layer ofthe UE transmits a GUMMEI to the AS layer. When receiving the GUMMEI,the AS layer transmits the GUMMEI to the eNB by including it in RA msg5[S1520]. Meanwhile, upon receiving the TAU request message of the remoteUE from the eNB, MME 1 needs to recognize that the corresponding NASmessage is made by the remote UE. This is because MME 1 should informthe relay UE that TAU accept/complete messages, which will be describedlater, will be transmitted to the remote UE. To this end, an indicatorinforming that the TAU request message is made by the remote UE isrequired. Such an indicator may be included in a NAS message (e.g., TAUrequest message) or an RRC message. In the former case, when the remoteUE creates the TAU request message, the remote UE may include thecorresponding indicator in the TAU request message. Alternatively, theremote UE may include the indicator in the PC5 message rather than theTAU request message and then transmit the indicator to the relay UE. Inthe latter case, the indicator may be included in an RRC connectionsetup complete message and an S1-AP message (e.g., initial UE message),which will be transmitted to the eNB, and then transmitted to MME 1.

Based on the information received in step S1515 c or S1520 (e.g., S-TMSIor MME indicator), the eNB transmits the TAU request message of theremote UE to MME 1, which corresponds to the MME of the relay UE.

Next, when receiving the TAU request message of the remote UE and theindicator indicating that the corresponding TAU request message is madeby the remote UE, MME 1 recognizes that the received TAU request messageis for the remote UE and then triggers a context request for the remoteUE according to the existing TAU procedure [S1530]. Specifically, MME 1derives the previous MME identifier (GUMMEI) from the GUTI of the remoteUE included in the TAU request message and request and receive contextinformation of the remote UE from MME 2, which corresponds to thecurrent MME of the remote UE [S1535 and S1540]. Then, the TAU proceduredescribed in TS 23.401 is sequentially performed by network entities[S1545]. When the TAU procedure is completed, MME 1 transmits the TAUaccept message to the relay UE [S1550], and the relay UE transmits theTAU complete message to MME 1 [S1555]. In this case, an indicatorindicating that corresponding NAS messages is for the remote UE may betransmitted together. The relay UE forwards the TAU accept message tothe remote UE through a PC5 message [S1560].

The relay UE-initiated method described with reference to FIG. 15 is amethod in which the relay UE transmits the TAU request message of theremote UE to the network instead of the remote UE. According to thismethod, the context information of the remote UE is transmitted to therelay UE, and MMEs of the remote UE and relay UE are aligned. Meanwhile,during the existing TAU procedure, a security process, an authenticationprocess, and a GUTI reallocation process may occur between a UE and thenetwork. When these processes are triggered, the network handle handlesthe processes by interacting with the remote UE through the relay UE.

FIGS. 16 and 17 illustrate MME alignment processes initiated by a remoteUE. In particular, FIG. 16 shows an eNB routing method, and FIG. 17shows an MME re-routing method.

Referring to FIG. 16, a relay UE transmits an S-TMSI of the relay UE oran MME identifier (i.e., GUMMEI) of MME 1 to a remote UE [S1605]. Inthis case, an indicator capable of extracting the S-TMSI (e.g., GUTI)may be transmitted instead of the S-TMSI of the relay UE or the MMEidentifier of MME 1. In addition, in the above step, a serving cell IDof the relay UE may also be transmitted to the remote UE together.

By performing a cell reselection procedure in consideration of theserving cell ID of the relay UE received in step S1605, the remote UEaligns its serving cell such that it becomes identical to that of therelay UE. Thereafter, the remote UE triggers a TAU procedure to alignits MME with that of the relay UE [S1610].

The remote UE transmits a TAU request message to an MME in which therelay UE is registered in (i.e., MME 1) using the identificationinformation of the relay UE received in step S1605 (e.g., S-TMSI, GUTI,or GUMMEI) [S1615 c and S1620]. This may be performed in such a mannerthat a NAS layer of a UE provides an AS layer with the S-TMSI or GUMMEIwhen a NAS signaling connection is established (for example, when a TAUprocedure is performed). Specifically, when a TA of the current cell isincluded in the list of TAs registered by the UE, the NAS layer of theUE transmits the S-TMSI to the AS layer. In addition, when the AS layeris provided with the S-TMSI by the NAS layer, the AS layer transmits theS-TMSI to the eNB by including it in RA msg3 [S1615 c]. On the otherhand, when the TA of the current cell is not included in the list of TAsregistered by the UE, the NAS layer of the UE transmits the GUMMEI tothe AS layer. When receiving the GUMMEI, the AS layer transmits theGUMMEI to the eNB by including it in RA msg5 [S1620]. In this case, theS-TMSI or GUMMEI transmitted by the remote UE to the eNB is not a thingmade by the remote UE but is the value received from the relay UE instep S1605. That is, the S-TMSI of the relay UE is transmitted to theeNB by being included in the RA msg3, or the GUMMEI of MME 1, which isthe MME of the relay UE, is transmitted to the eNB by being included inthe RA msg5.

The eNB forwards the TAU request message to identified MME 1 based onthe S-TMSI of the relay UE or the MME identifier of MME 1 [S1625], andMME 1 receives context information of the remote UE from MME 2 bytriggering a context request procedure of the remote UE [S1630, S1635,and S1640]. Thereafter, the TAU procedure described in TS 23.401 issequentially performed by network entities [S1645]. When the TAUprocedure is completed, MME 1 transmits a TAU accept message to theremote UE [S1650], and the remote UE transmits a TAU complete message toMME 1 [S1655].

Referring to FIG. 17, a relay UE transmits identification information(e.g., GUMMEI) of MME 1 to a remote UE through a PC5 message [S1705]. Inthis case, an identifier (e.g., GUTI) capable of extracting the MMEidentifier may be transmitted instead of the MME identifier. The remoteUE triggers a TAU procedure based on the PC5 message received from therelay UE [S1710]. The remote UE forwards, to the network (MME 2) theGUMMEI of MME 1, which is an MME of the relay UE, received in step S1705by including it in a TAU request message [S1720]. In this case, the TAUrequest message may additionally include an indicator for triggering MMEre-routing.

Thereafter, eNB 1 forwards the TAU request message to an MME of theremote UE, MME 2 [S1725]. In this case, the TAU request message mayinclude an indicator indicating MME re-routing together with theidentifier of MME 1. When receiving the TAU request message and checkingthe identifier of MME 1 (or the indicator indicating MME re-routing),MME 2 decides to re-route the TAU request message and then triggers aTAU procedure for re-routing to MME 1 [S1730]. Then, MME 2 transmits, tothe eNB, the identifier of MME 1 together with the TAU request messageby including the identifier in an S1AP message for the purpose of MMEre-routing [S1735]. After receiving the S1AP message, the eNB checks MME1 based on the identifier of MME 1 included in the S1AP message [S1740]and then transmits the TAU request message to MME 1 [S1745]. To alignthe MME of the remote UE, MME 1 requests context information of theremote UE from MME 2 and then receive a response [S1750 and S1755]. Theremaining TAU procedure may be applied in a similar manner as stepsS1645 to 1655 described in FIG. 16.

5. Device Configurations

FIG. 18 is a diagram illustrating configurations of node devicesaccording to a proposed embodiment.

A user equipment (UE) 100 may include a transceiver 110, a processor120, and a memory 130. The transceiver 110 may be configured to transmitand receive various signals, data, and information to/from an externaldevice. Alternatively, the transceiver 110 may be implemented with acombination of a transmitter and a receiver. The UE 100 may be connectedto the external device by wire and/or wirelessly. The processor 120 maybe configured to control overall operations of the UE 100 and processinformation to be transmitted and received between the UE 100 and theexternal device. Moreover, the processor 120 may be configured toperform the UE operation proposed in the present invention. The memory130, which may be replaced with an element such as a buffer (not shownin the drawing), may store the processed information for a predeterminedtime.

Referring to FIG. 18, a network node 200 according to the presentinvention may include a transceiver 210, a processor 220, and a memory230. The transceiver 210 may be configured to transmit and receivevarious signals, data, and information to/from an external device. Thenetwork node 200 may be connected to the external device by wire and/orwirelessly. The processor 220 may be configured to control overalloperations of the network node 200 and process information to betransmitted and received between the network node device 200 and theexternal device. Moreover, the processor 220 may be configured toperform the network node operation proposed in the present invention.The memory 230, which may be replaced with an element such as a buffer(not shown in the drawing), may store the processed information for apredetermined time.

The specific configurations of the UE 100 and the network node 200 maybe implemented such that the aforementioned various embodiments of thepresent invention can be independently applied, or two or moreembodiments can be simultaneously applied. For clarity, redundantdescription will be omitted.

The embodiments of the present invention may be implemented usingvarious means. For instance, the embodiments of the present inventionmay be implemented using hardware, firmware, software and/or anycombinations thereof.

In case of the implementation by hardware, a method according to eachembodiment of the present invention may be implemented by at least oneselected from the group consisting of ASICs (application specificintegrated circuits), DSPs (digital signal processors), DSPDs (digitalsignal processing devices), PLDs (programmable logic devices), FPGAs(field programmable gate arrays), processor, controller,microcontroller, microprocessor and the like.

In case of the implementation by firmware or software, a methodaccording to each embodiment of the present invention can be implementedby modules, procedures, and/or functions for performing theabove-explained functions or operations. Software code may be stored ina memory unit and be then executed by a processor. The memory unit maybe provided within or outside the processor to exchange data with theprocessor through the various means known to the public.

As mentioned in the foregoing description, the detailed descriptions forthe preferred embodiments of the present invention are provided to beimplemented by those skilled in the art. While the present invention hasbeen described and illustrated herein with reference to the preferredembodiments thereof, it will be apparent to those skilled in the artthat various modifications and variations can be made therein withoutdeparting from the spirit and scope of the invention. Therefore, thepresent invention is non-limited by the embodiments disclosed herein butintends to give a broadest scope matching the principles and newfeatures disclosed herein.

INDUSTRIAL APPLICABILITY

The aforementioned direct communication method can be applied to notonly the 3GPP system but also various wireless communication systemsincluding an IEEE 802.16x system and IEEE 802.11x system. Further, theproposed method can also be applied to an mmWave communication systemusing super-high frequency band.

1-16. (canceled)
 17. A method performed by a new mobility managemententity (MME) supporting tracking area update (TAU) procedure of a relayuser equipment (UE), connected with one or more remote UE correspondingto a Proximity Service-enabled (ProSe-enabled) UE, in a wirelesscommunication system, the method comprising: receiving a TAU requestmessage from the relay UE; transmitting a context request message to anold MME associated with the relay UE, according to the TAU procedure;and receiving a context response message comprising UE contextcorresponding to the relay UE and the one or more remoted UE from theold MME, in response to the context response message.
 18. The method ofclaim 17, wherein the TAU request message comprises a group identity ofa group comprising the relay UE and the one or more remote UE oridentities of UEs belong to the group.
 19. The method of claim 17,wherein the context request message is transmitted with information forrequesting transmission of the UE context corresponding to the relay UEand the one or more remoted UE included.
 20. A method performed by anold mobility management entity (MME) supporting tracking area update(TAU) procedure of a relay user equipment (UE), connected with one ormore remote UE corresponding to a Proximity Service-enabled(ProSe-enabled) UE, in a wireless communication system, the methodcomprising: receiving a context request message from a new MMEassociated with the relay UE, according to the TAU procedure; andtransmitting a context response message comprising UE contextcorresponding to the relay UE and the one or more remoted UE to the newUE, in response to the context response message.
 21. The method of claim20, wherein the context request message is received with information forrequesting transmission of the UE context corresponding to the relay UEand the one or more remoted UE included.
 22. A new mobility managemententity (MME), associated with a relay user equipment (UE), supportingtracking area update (TAU) procedure of the relay UE, connected with oneor more remote UE corresponding to a Proximity Service-enabled(ProSe-enabled) UE, in a wireless communication system, the new MMEcomprising: a transmitter; a receiver; and a processor connected to thetransmitter and the receiver, wherein the processor: receives a TAUrequest message from the relay UE; transmits a context request messageto an old MME associated with the relay UE, according to the TAUprocedure; and receives a context response message comprising UE contextcorresponding to the relay UE and the one or more remoted UE from theold MME, in response to the context response message.
 23. The new MME ofclaim 22, wherein the TAU request message comprises a group identity ofa group comprising the relay UE and the one or more remote UE oridentities of UEs belong to the group.
 24. The new MME of claim 22,wherein the context request message is transmitted with information forrequesting transmission of the UE context corresponding to the relay UEand the one or more remoted UE included.
 25. An old mobility managemententity (MME), associated with a relay user equipment (UE), supportingtracking area update (TAU) procedure of the relay UE, connected with oneor more remote UE corresponding to a Proximity Service-enabled(ProSe-enabled) UE, in a wireless communication system, the old MMEcomprising: receiving a context request message from a new MMEassociated with the relay UE, according to the TAU procedure; andtransmitting a context response message comprising UE contextcorresponding to the relay UE and the one or more remoted UE to the newUE, in response to the context response message. a transmitter; areceiver; and a processor connected to the transmitter and the receiver,wherein the processor: receives a context request message from a new MMEassociated with the relay UE, according to the TAU procedure; andtransmits a context response message comprising UE context correspondingto the relay UE and the one or more remoted UE to the new UE, inresponse to the context response message.
 26. The old MME of claim 25,wherein the context request message is received with information forrequesting transmission of the UE context corresponding to the relay UEand the one or more remoted UE included.